The present invention relates generally to improved adhesive compositions. More particularly, the present invention relates to improved adhesive compositions containing low molecular weight, polymeric additives and methods for forming the same.
The properties of tack, peel strength and shear resistance, which are frequently mutually exclusive properties, may be highly important in tailoring an adhesive composition that is suitable for a particular application. Tack is generally a measure of viscous flow under conditions of fast strain rates and low stress magnitudes. Peel strength is a measure of resistance to flow at intermediate strain rates and moderate to high stress magnitudes, as well as the cohesive strength of the adhesive. Shear resistance is a measure of resistance to flow at intermediate stress magnitudes.
Polymeric additives, such as tackifiers or plasticizers, are often added to adhesive compositions to modify the properties of the adhesive including, for example, the viscoelastic behavior of the finished adhesive. The particular polymeric additive selected may vary depending, for example, on the adhesive involved and the modification sought.
Traditionally, natural and synthetic rubbers have been used as pressure-sensitive adhesives, but these rubbers alone may not have the necessary balance of properties of tack, shear, and peel adhesion. For example, rubber-based adhesives typically require the addition of tackifiers, such as rosin acid derivatives or aliphatic/aromatic resins, to impart useful properties as pressure-sensitive adhesives.
Acrylic polymers have also been useful as pressure-sensitive adhesives. Acrylic polymers have typically not required compounding with tackifier resins to achieve desirable pressure sensitive characteristics. Recently, however, it has been shown that tackifiers may provide a convenient and useful tool for modifying pressure-sensitive adhesives as discussed by T. G. Wood, xe2x80x9cThe Effects of Tackification on Waterborne Acrylic Pressure-Sensitive Adhesivexe2x80x9d, Adhesive Age, Vol. 30, pp. 19-23 (1987). The Handbook of Pressure-Sensitive Adhesive Technology, edited by Don Satas, Van Nostrand Reinhold Co., pp 353-369, (1982) provides a discussion on pressure-sensitive adhesives. This reference teaches, inter alia, that the addition of tackifiers to acrylic, pressure-sensitive adhesive polymers may improve the performance characteristics for these materials.
Generally speaking, to enhance performance characteristics of adhesive compositions, the tackifier should possess a certain range of properties with respect to the adhesive polymer or copolymer to which it is added. For example, the tackifier should generally have a lower molecular weight as compared to that of the adhesive polymer or copolymer. In addition, the tackifier should generally have a glass transition temperature (Tg) that is higher than that of the adhesive polymer or copolymer. The softening temperature of the tackifier should generally be higher than that of the adhesive polymer or copolymer, and the tackifier should generally have a minimum degree of compatibility with the adhesive polymer or copolymer.
Plasticizers may be added to an adhesive composition, for example, to increase its workability, flexibility, and/or distensibility. Depending upon the involved adhesive system, the addition of a plasticizer may lower the melt viscosity, elastic modulus, and glass transition temperature (Tg) of the overall system. Plasticizers may also be added to polymer formulations to increase flexibility by promoting bond formation between polymer molecules and the plasticizer rather than between the polymer molecules themselves. In doing so, the plasticizer may also increase the specific volume and change the index of refraction of the resulting composition. The addition of the plasticizer to the adhesive composition may also raise the viscosity of the adhesive system.
Japanese Patent Disclosure No. 54-3136 discloses a pressure-sensitive adhesive containing an acrylic polymer and a tackifier. The tackifier is prepared by solution polymerizing 25 to 75 weight % of vinyl aromatic compound and 75 to 25 weight % acrylic or methacrylic acid ester. The disclosed tackifier has a number average molecular weight of 500 to 3,000 and a softening point of less than 40xc2x0 C.
U.S. Pat. No. 3,867,481 discloses a low molecular weight acrylic polymer containing (1) an alkyl methacrylate and (2) one or more additional acrylic methacrylic acid ester and/or a styrene for use as a processing modifier for rigid polyvinyl halide resins.
U.S. Pat. No. 4,056,559 discloses low molecular weight polymers of alkyl methacrylates having an average chain length of about 6 to about 50 mers and having a variety of uses in films, coatings, adhesives and inks.
U.S. Pat. No. 4,912,169 discloses low molecular weight tackifiers that have a number average molecular weight less than about 35,000 and a softening point greater than about 40xc2x0 C. The tackifiers are prepared with high levels, or 1 to 20 mole percent, of chain transfer agents such as C1-C15 alkyl mercaptans, benzyl mercaptan, 3-mercaptoproprionic acid and esters thereof, mercaptoethanol, benzyl alcohol, alpha-methyl benzyl alcohol, and ethyl mercaptoacetate.
Mercaptans are often effective for use as chain transfer agents and in reducing the molecular weight of polymers. Such uses of mercaptans is problematic, however, due to, inter alia, the expense of these materials and their frequently offensive odors. Moreover, the use of mercaptans during processing can impart thiol functionalities into the resulting polymer, thereby affecting the properties of the polymer. The use of other common chain transfer agents such as, for example, hypophosphites, sulfates, and alcohols, may also be undesirable in that they may add to the cost of the process, impart undesired functionality to the polymer, introduce undesired salts into the process, and introduce additional process steps, including product separation.
The present invention seeks to provide improved adhesive compositions containing polymeric additives for use, for example, as adhesives. The polymeric additives of the present invention can be prepared without the use of high levels of chain transfer agents. Moreover, the polymeric additives of the present invention can be prepared without mercaptans or alcohols as chain transfer agents.
The present invention is directed, in part, to improved adhesive compositions. Specifically, in one embodiment, there are provided adhesive compositions comprising (a) adhesive polymers or copolymers prepared from monomers selected from the group consisting of styrene, butadiene, acrylonitrile, ethylene, vinyl acetate, acrylic acid, esters of acrylic acid, methacrylic acid, esters of methacrylic acid and combinations thereof; and (b) from 1 to 55% on a dry weight basis of a polymeric additive comprising a low molecular weight polymer having a number average molecular weight of 20,000 or less and which comprises: (i) polymerized units of at least one ethylenically unsaturated monomer; and (ii) 5 weight percent or less of a polymerized acid-containing monomer wherein the polymeric additive is polymerized in the presence of a metal chelate chain transfer agent.
Another aspect of the invention relates to a method of modifying one or more properties of an adhesive comprising combining with the adhesive polymer or copolymer from 1 to 55% on a dry weight basis of a polymeric additive comprising a low molecular weight polymer having a number average molecular weight of 20,000 or less and which comprises: (i) polymerized units of at least one ethylenically unsaturated monomer; and (ii) 5 weight percent or less of a polymerized acid-containing monomer wherein the polymeric additive is polymerized in the presence of a metal chelate chain transfer agent.
Still another aspect of the invention relates to methods for the in-situ generation of a polymeric additive within an adhesive composition, the method comprising: (a) polymerizing at least one first ethylenically unsaturated monomer to form an adhesive polymer or copolymer; (b) polymerizing, in the presence of said adhesive polymer or copolymer, a polymeric additive comprising a low molecular weight polymer having a number average molecular weight of 20,000 or less and which comprises: (i) polymerized units of at least one second ethylenically unsaturated monomer; and (ii) 5 weight percent or less of a polymerized acid-containing monomer, wherein said polymeric additive is formed in the presence of a metal chelate chain transfer agent.
These and other aspects of the invention will become more apparent from the following detailed description.
The present invention is directed to improved adhesive compositions containing relatively low molecular weight polymer additives. The present adhesive compositions exhibit an improved balance of properties in comparison to adhesive compositions of the prior art. In particular, the adhesive compositions of this invention may exhibit an improved balance of tack, peel strength and shear resistance, preferably without the problems in clarity, color, and UV resistance caused when typical rosin or hydrocarbon tackifiers are used. In addition, in certain embodiments, the present invention also provides methods of preparing adhesive compositions wherein the polymeric additive is generated in situ during the formation of the adhesive polymer or copolymer.
In certain preferred embodiments, the low molecular weight polymer is in the form of a macromonomer. The term xe2x80x9cmacromonomerxe2x80x9d, as used herein, refers to any low molecular weight water-insoluble polymer or copolymer that may be capable of being polymerized in a free radical polymerization process. The term xe2x80x9cwater-insolublexe2x80x9d, as used herein, means that the macromonomer has a water solubility of no greater than 150 millimoles/liter at 25xc2x0 C. to 50xc2x0 C. The term xe2x80x9clow molecular weightxe2x80x9d, as used herein, means that the macromonomer has a number average molecular weight of 20,000 or less. More preferably, the number average molecular weight of the macromonomer is 9,000 or less. Even more preferably, the number average molecular weight of the macromonomer is from m 600 to 6000, with number average molecular weights of from 600 to 4000 being still more preferred.
The low molecular weight polymer contains, as polymerized units, at least one type of ethylenically unsaturated monomer. Preferably, the ethylenically unsaturated monomer is selected to impart low or no water solubility to the low molecular weight polymer as previously described herein.
Suitable ethylenically unsaturated monomers for use in preparing the low molecular weight polymers employed in the methods and compositions of the present invention include, for example, methacrylate esters, such as C1 to C18 normal or branched alkyl esters of methacrylic acid, including methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, lauryl methacrylate, stearyl methacrylate; acrylate esters, such as C1 to C18 normal or branched alkyl esters of acrylic acid, including methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate; styrene; substituted styrenes, such as methyl styrene, xcex1-methyl styrene or t-butyl styrene; olefinically unsaturated nitriles, such as acrylonitrile or methacrylonitrile; olefinically unsaturated halides, such as vinyl chloride, vinylidene chloride or vinyl fluoride; vinyl esters of organic acids, such as vinyl acetate; N-vinyl compounds such as N-vinyl pyrrolidone; acrylamide; methacrylamide; substituted acrylamides; substituted methacrylamides; hydroxyalkylmethacrylates such as hydroxyethylmethacrylate; hydroxyalkylacrylates; basic substituted (meth)acrylates and (meth)acrylamides, such as amine-substituted methacrylates including dimethylaminoethyl methacrylate, tertiary-butylaminoethyl methacrylate and dimethylaminopropyl methacrylamide and the likes; dienes such as 1,3-butadiene and isoprene; vinyl ethers; or combinations thereof. The term xe2x80x9c(meth)xe2x80x9d, as used herein, means that the xe2x80x9cmethxe2x80x9d is optionally present. For example, xe2x80x9c(meth)acrylatexe2x80x9d means methacrylate or acrylate.
The ethylenically unsaturated monomer can also be a functional monomer including, for example, monomers containing such functional groups as hydroxy, amido, aldehyde, ureido, polyether, glycidylalkyl, keto and combinations thereof. These functional monomers are generally present in the low molecular weight polymer at a level of from 0.5 weight percent to 15 weight percent and more preferably from 1 weight percent to 3 weight percent, based on the total weight of the low molecular weight polymer. Examples of functional monomers include, for example, ketofunctional monomers, such as the acetoacetoxy esters of hydroxyalkyl acrylates and methacrylates (e.g., acetoacetoxyethyl methacrylate) and keto-containing amides (e.g., diacetone acrylamide); allyl alkyl methacrylates or acrylates; glycidylalkyl methacrylates or acrylates; or combinations thereof. Such functional monomers can provide crosslinking, if desired.
The low molecular weight polymer also contains as polymerized units 10 weight percent or less, preferably 5 weight percent or less, more preferably 2 weight percent or less and most preferably 1 weight percent or less acid containing monomer, based on the total weight of the low molecular weight polymer. In a particularly preferred embodiment, the low molecular weight polymer contains no acid containing monomer. The term xe2x80x9cacid containing monomerxe2x80x9d, as used herein, refers to any ethylenically unsaturated monomer that contains one or more acid functional groups or functional groups that are capable of forming an acid (e.g., an anhydride such as methacrylic anhydride or tertiary butyl methacrylate). Examples of acid containing monomers include, for example, carboxylic acid bearing ethylenically unsaturated monomers such as acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid; acryloxypropionic acid and (meth)acryloxypropionic acid; sulphonic acid-bearing monomers, such as styrene sulfonic acid, sodium vinyl sulfonate, sulfoethyl acrylate, sulfoethyl methacrylate, ethylmethacrylate-2-sulphonic acid, or 2-acrylamido-2-methylpropane sulphonic acid; phosphoethylmethacrylate; the corresponding salts of the acid containing monomer; or combinations thereof.
As polymerized, the low molecular weight polymer is substantially (including completely) free of mercapto-olefin compounds. The term xe2x80x9csubstantially freexe2x80x9d, as used herein, means that the low molecular weight polymer contains, as polymerized, less than 5 mole percent mercapto-olefin compounds, based on the total weight of the low molecular weight polymer. Preferably, the macromomer contains less than 2 mole percent mercapto-olefin compounds, with less than 1 mole percent being more preferred. Even more preferably, the low molecular weight polymer contains less than 0.5 mole percent mercapto-olefin compounds. In certain particularly preferred embodiments, the low molecular weight polymer contains completely no (i.e., 0 mole percent) mercapto-olefin compounds. Some examples of mercapto-olefin compounds are those compounds described in U.S. Pat. No. 5,247,000 to Amick. The mercapto-olefin compounds described in Amick have ester functional groups, which are susceptible to hydrolysis.
In a preferred embodiment of the present invention, the low molecular weight polymer is composed of at least 20 weight percent, more preferably from 50 weight percent to 100 weight percent, and even more preferably from 80 to 100 weight percent of at least one xcex1-methyl vinyl monomer, a non xe2x80x3xcex1-methyl vinyl monomer terminated with a xe2x80x3xcex1-methyl vinyl monomer, or combinations thereof. In a particularly preferred embodiment of the present invention, the low molecular weight polymer contains as polymerized units from 90 weight percent to 100 weight percent xcex1-methyl vinyl monomers, non xe2x80x3xcex1-methyl vinyl monomers terminated with xe2x80x3xcex1-methyl vinyl monomers, or combinations thereof, based on the total weight of the low molecular weight polymer. Suitable xcex1-methyl vinyl monomers include, for example, methacrylate esters, such as C1 to C18 normal or branched alkyl esters of methacrylic acid, including methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, lauryl methacrylate, or stearyl methacrylate; hydroxyalkyl methacrylates such as hydroxyethyl methacrylate; glycidylmethacrylate; phenyl methacrylate; methacrylamide; methacrylonitrile; or combinations thereof. An example of a non xe2x80x3xcex1-methyl vinyl monomer terminated with an xe2x80x3xcex1-methyl vinyl monomer includes styrene terminated by xe2x80x3xcex1-methyl styrene.
The low molecular weight polymers useful in the present invention may be prepared by polymerizing ethylenically unsaturated monomers in the presence of a free radical initiator and a catalytic metal chelate chain transfer agent (e.g., a transition metal chelate). Such a polymerization may be carried out by a solution, bulk, suspension, or emulsion polymerization process. Suitable methods for preparing the low molecular weight polymer using a catalytic metal chelate chain transfer agent are disclosed, for example, in U.S. Pat. Nos. 4,526,945, 4,680,354, 4,886,861, 5,028,677, 5,362,826, 5,721,330, and 5,756,605; European publications EP-A-0199,436, and EP-A-0196783; and PCT publications WO 87/03605, WO 96/15158, and WO 97/34934.
In accordance with preferred embodiments of the present invention, the low molecular weight polymer may be prepared by an aqueous emulsion free radical polymerization process. It has been surprisingly and unexpectedly found that this polymerization process may be advantageously carried out without the use of prior art chain transfer agents such as, for example, mercaptans, hypophosphites, sulfates, and alcohols. Such prior art chain transfer agents may be undesirable in that they may exhibit offensive odors that may be imparted to the polymer, and may also add to the cost of the process, impart undesired functionality to the polymer, introduce undesired salts into the process, and introduce additional process steps, including product separation.
The aqueous emulsion free radical polymerization process is preferably conducted using a transition metal chelate complex as a chain transfer agent. Preferably, the transition metal chelate complex is a cobalt (II) or (III) chelate complex such as, for example, dioxime complexes of cobalt, cobalt II porphyrin complexes, or cobalt II chelates of vicinal iminohydroxyimino compounds, dihydroxyimino compounds, diazadihydroxy-iminodialkyldecadienes, or diazadihydroxyiminodialkylundecadienes, or combinations thereof. These complexes may optionally include bridging groups such as BF2, and may also be optionally coordinated with ligands such as water, alcohols, ketones, and nitrogen bases such as pyridine. Additional suitable transition metal complexes are disclosed, for example, in U.S. Pat. Nos. 4,694,054; 5,770,665; 5,962,609; and 5,602,220. A preferred cobalt chelate complex useful in the preparation of the low molecular weight polymers of the present invention is Co II (2,3-dioxyiminobutane-BF2)2, the Co III analogue of the aforementioned compound, or combinations thereof. The spatial arrangements of such complexes are disclosed, for example, in EP-A-199436 and U.S. Pat. No. 5,756,605.
In preparing the low molecular weight polymer by an aqueous emulsion polymerization process using a transition metal chelate chain transfer agent, at least one ethylenically unsaturated monomer is polymerized in the presence of a free radical initiator and the transition metal chelate according to conventional aqueous emulsion polymerization techniques. Preferably, the ethylenically unsaturated monomer is an xcex1-methyl vinyl monomer as previously described herein.
The polymerization reaction to form the low molecular weight polymer is preferably conducted at a temperature of from 20xc2x0 C. to 150xc2x0 C., and more preferably from 40xc2x0 C. to 95xc2x0 C. The solids level at the completion of the polymerization is typically from 5 weight percent to 65 weight percent, and more preferably from 30 weight percent to 55 weight percent, based on the total weight of the aqueous emulsion.
The concentration of initiator and transition metal chelate chain transfer agent used during the polymerization process is preferably chosen to obtain the desired degree of polymerization of the low molecular weight polymer. Preferably, the concentration of initiator is from 0.2 weight percent to 3 weight percent, and more preferably from 0.5 weight percent to 1.5 weight percent, based on the total weight of monomer. Preferably, the concentration of transition metal chelate chain transfer agent is from 5 ppm to 200 ppm, and more preferably from 10 ppm to 125 ppm, based on the total moles of monomer used to form the low molecular weight polymer.
The ethylenically unsaturated monomer, initiator, and transition metal chelate chain transfer agent may be combined in any manner known to those skilled in the art to carry out the polymerization. For example, the monomer, initiator and transition metal chelate may all be present in the aqueous emulsion at the start of the polymerization process (i.e., a batch process). Alternatively, one or more of the components may be gradually fed to an aqueous solution (i.e., a continuous or semi-batch process). For example, it may be desired to gradually feed the entire or a portion of the initiator, monomer, and/or transition metal chelate to a solution containing water and surfactant. In a preferred embodiment, at least a portion of the monomer and transition metal chelate are gradually fed during the polymerization, with the remainder of the monomer and transition metal chelate being present in the aqueous emulsion at the start of the polymerization. In this embodiment, the monomer may be fed as is, or suspended or emulsified in an aqueous solution prior to being fed.
Any suitable free radical initiator may be used to prepare the low molecular weight polymer. The initiator is preferably selected based on such parameters as its solubility in one or more of the other components (e.g., monomers, water); half life at the desired polymerization temperature (preferably a half life within the range of from 30 minutes to 10 hours), and stability in the presence of the transition metal chelate. Suitable initiators include for example azo compounds such as 2,2xe2x80x2-azobis (isobutyronitrile), 4,4xe2x80x2-azobis(4-cyanovaleric acid), 2,2xe2x80x2-azobis [2-methyl-N-(1,1-bis(hydroxymethyl)-2-(hydroxyethyl)]-propionamide, and 2,2xe2x80x2-azobis [2-methyl-N-(2-hydroxyethyl)]-propionamide; peroxides such as t-butyl hydroperoxide, benzoyl peroxide; sodium, potassium, or ammonium persulphate or combinations thereof. Redox initiator systems may also be used, such as for example persulphate or peroxide in combination with a reducing agent such as sodium metabisulphite, sodium bisulfite, sodium formaldehyde sulfoxylate, isoascorbic acid, or combinations thereof. Metal promoters, such as iron, may also optionally be used in such redox initiator systems. Also, buffers, such as sodium bicarbonate may be used as part of the initiator system.
An emulsifier is also preferably present during the aqueous emulsion polymerization process to prepare the low molecular weight polymer. Any emulsifier may be used that is effective in emulsifying the monomers such as for example anionic, cationic, or nonionic emulsifiers. In a preferred embodiment, the emulsifier is anionic such as for example sodium, potassium, or ammonium salts of dialkylsulphosuccinates; sodium, potassium, or ammonium salts of sulphated oils; sodium, potassium, or ammonium salts of alkyl sulphonic acids, such as sodium dodecyl benzene sulfonate; sodium, potassium, or ammonium salts of alkyl sulphates, such as sodium lauryl sulfate; ethoxylated alkyl ether sulfates; alkali metal salts of sulphonic acids; C12 to C24 fatty alcohols, ethoxylated fatty acids or fatty amides; sodium, potassium, or ammonium salts of fatty acids, such as Na stearate and Na oleate; or combinations thereof. The amount of emulsifier in the aqueous emulsion is preferably from 0.05 weight percent to 10 weight percent, and more preferably from 0.3 weight percent to 3 weight percent, based on the total weight of the monomers.
The low molecular weight polymer aqueous emulsion useful in the present invention may be formed in any manner known to those skilled in the art. For example, the low molecular weight polymer, produced by any known method, may be isolated as a solid and emulsified in water. Also, for example, the low molecular weight polymer, if prepared via an emulsion or aqueous based polymerization process, may be used as is, or diluted with water or concentrated to a desired solids level.
In a preferred embodiment of the present invention, the low molecular weight polymer aqueous emulsion is formed from the emulsion polymerization of an ethylenically unsaturated monomer in the presence of a transition metal chelate chain transfer agent as described previously herein. This embodiment is preferred for numerous reasons. For example, the low molecular weight polymer polymerization can be readily controlled to produce a desired particle size distribution (preferably narrow, e.g., polydispersity less than 2). Also, for example, additional processing steps, such as isolating the low molecular weight polymer as a solid, can be avoided, leading to better process economics. In addition, the low molecular weight polymer, low molecular weight polymer aqueous emulsion and the adhesive composition may be prepared by consecutive steps in a single reactor which is desirable in a commercial manufacturing facility.
The low molecular weight polymer aqueous emulsion useful in the present invention contains from 20 weight percent to 60 weight percent, and more preferably from 30 weight percent to 50 weight percent of at least one water insoluble low molecular weight polymer, based on the total weight of low molecular weight polymer aqueous emulsion. The low molecular weight polymer aqueous emulsion may also contain mixtures of low molecular weight polymer. Preferably, the low molecular weight polymer aqueous emulsion contains 5 weight percent or less, and more preferably 1 weight percent or less of ethylenically unsaturated monomer, based on the total weight of low molecular weight polymer aqueous emulsion.
The water insoluble low molecular weight polymer particles have a particle size that allows it to be compatible to the adhesive compositions of the desired particle size. Preferably, the low molecular weight polymer particles have a weight average particle size of from 50 nm to 600 nm, and more preferably from 80 nm to 200 nm as measured by Capillary Hydrodynamic Fractionation technique using a Matec CHDF 2000 particle size analyzer equipped with a HPLC type Ultra-violet detector.
The low molecular weight polymer aqueous emulsion may also include one or more emulsifying agents. The type and amount of emulsifying agent is preferably selected in a manner to produce the desired particle size. Suitable emulsifying agents include those previously disclosed for use in preparing the low molecular weight polymer by an emulsion polymerization process. Preferred emulsifying agents are anionic surfactants such as, for example, sodium lauryl sulfate, sodium dodecylbenzene sulfonate, sulfated and ethoxylated derivatives of nonylphenols and fatty alcohols. The total level of emulsifying agent, based on the total weight of low molecular weight polymer is preferably from 0.2 weight percent to 5 weight percent and more preferably from 0.5 weight percent to 2 weight percent.
The ability of a polymeric additive to improve the performance of adhesive compositions may depend, for example, on the molecular weight of the polymeric additive, or the degree of compatibility between the additive and the adhesive polymer or copolymer. Compatibility, as used herein, refers to a measure of the mutual solubility of the two materials. Compatible blends may be characterized by (1) the existence of a single homogeneous phase which contains no discrete domains of either component, and (2) a single glass transition temperature for the blend of components as discussed. See, e.g., P. B. Rim and E. B. Orler, xe2x80x9cDependence of Tg on Composition for a Compatible Polymer/Oligomer Blendxe2x80x9d, Macromolecules, Vol. 20, pp. 433-435 (1987). A further aspect of compatibility relates to the differences in refractive indices between the polymeric additive and the adhesive polymer or copolymer. A lack of compatibility is generally evidenced by haziness in the dried film (which shows different refractive indices) and a decrease in tack. Compatibility is typically favored between materials that are similar in chemical and/or physical characteristics. To effectively modify the performance of an adhesive, the polymeric additives preferably demonstrate an appreciable degree of compatibility with the adhesive polymer, although complete compatibility is not necessary. It is preferable, however, that the polymeric additive and the adhesive polymer be at least partially compatible.
It is generally desirable to optimize the balance of properties of an adhesive such as peel strength, shear strength, tack, and film clarity. All of these properties are important to adhesive compositions and the best advantages are achieved where the overall balance of these properties have been optimized. To achieve the necessary balance of adhesive properties, the polymeric additive of this invention preferably has a number average molecular weight of 20,000 or less. More preferably, the number average molecular weight of the polymeric additive is 9,000 or less. Even more preferably, the number average molecular weight of the polymeric additive is from 600 to 6000, with number average molecular weights of from 600 to 4000 being still more preferred.
In embodiments in which the polymeric additive is a tackifier, desirably enhanced peel strength and tack of the adhesive composition may be achieved when the polymeric additive preferably has a midpoint glass transition temperature (Tg) of 0xc2x0 C. or greater. The Tg values set forth herein are based on measured values obtained, for example, by differential scanning calorimetry of the respective polymer. On the other hand, polymeric additives of the present invention with a midpoint Tg below 0xc2x0 C. will work effectively as plasticizers in the adhesive composition.
The polymeric additive of this invention can be added to the adhesive polymers or copolymers by any suitable technique, such as by mixing or blending, to uniformly incorporate the additive into the adhesive compositions. The additive is preferably added to the adhesive composition in the form of an aqueous emulsion or an emulsified solution. In more preferred embodiments, the adhesive composition of the present invention is made by adding the polymeric additive and any other components of the adhesive composition to an aqueous emulsion containing the adhesive polymer or copolymer and agitating the combination to form a substantially uniform emulsion. The polymeric additives of the present invention can also be added to adhesive polymers involving solvents or solids, such as hot melt systems. An effective amount of the polymeric additive typically ranges from 1% to 55%, preferably 2.5% to 40%, and more preferably 7.5% to 30%, by weight based on the total dry weight of the adhesive composition.
The polymeric additives of the present invention may be used in a variety of adhesive compositions, including, but not limited to, pressure sensitive adhesives, construction adhesives, contact adhesives, hot melts, or solvent-based adhesive systems. The adhesive compositions may contain rubbers, such as isoprene and acrylonitrile-butadiene-styrene and polymers or copolymers prepared from styrene, butadiene, ethylene, vinyl acetate, acrylic acid or esters thereof, methacrylic acid or esters, or combinations thereof. Preferably, the adhesive composition comprises polymers or copolymers prepared from acrylic or methacrylic acid or esters thereof.
The polymeric additives of this invention can also be used with pressure sensitive adhesives comprising graft copolymers including, for example, the graft copolymers described in copending U.S. application, filed concurrently herewith, and assigned to the same entity of the present invention.
In certain embodiments, the adhesive compositions of the present invention, which comprise a polymeric additive, may be conveniently and advantageously prepared in situ with an adhesive polymer or copolymer using a two-stage process. In certain preferred embodiments, a first monomer solution is provided that comprises at least one ethylenically unsaturated monomer. A portion of the first ethylenically unsaturated monomer is polymerized to form a polymeric adhesive. In the second stage, a second monomer solution, that also comprises at least one ethylenically unsaturated monomer, is provided and is preferably polymerized in the presence of a metal chelate chain transfer agent to provide a low molecular weight polymer that acts as an additive (for example as a tackifier) for the first stage adhesive polymer.
In an alternative embodiment of the present invention, in the first stage, the first monomer solution is provided that comprises at least one ethylenically unsaturated monomer and is preferably polymerized in the presence of a metal chelate chain transfer agent to provide a polymeric additive that comprises a low molecular weight polymer. In the second stage, a second monomer solution is provided that also comprises at least one ethylenically unsaturated monomer and is preferably polymerized to form a polymeric adhesive.
In embodiments where the polymeric additive is prepared in the first stage, the monomers within the low molecular weight polymer are preferably selected to minimize reactivity of any unsaturation introduced by the metal chelate chain transfer agent. The term xe2x80x9creactivityxe2x80x9d, as used herein, relates to the reactivity ratio, or the ability to produce homopolymers or copolymers. Ethylenically unsaturated monomers which have greater reactivity include, for example, monomers in which the double bond is activated, for example, as in the case of methylacrylate, methyl methacrylate and vinyl acetate. Ethylenically unsaturated monomers which have lower reactivity include, for example, monomers in which the double bond is not activated, for example, as in the case of styrene. In addition, the reactivity of any particular monomer, relative to other monomers, is generally reduced as the substitution on the unsaturated carbon atoms increases. For example, in certain embodiments, styrene may preferably be selected as the monomer within the low molecular weight polymer emulsion in the first stage because unsaturation generated within this polymer from the metal chelate chain transfer agent has a lessened tendency to polymerize with the second stage adhesive composition and remains as a low molecular weight polymeric additive.
For efficiency, these two stages may preferably be conducted in a single vessel to prepare the adhesive formulation. For example, in the first stage, the low molecular weight polymer aqueous emulsion may be formed by polymerizing in an aqueous emulsion at least one first ethylenically unsaturated monomer to form water insoluble low molecular weight polymer particles. This first stage polymerization is preferably conducted using a transition metal chelate chain transfer agent as previously described herein. After forming the low molecular weight polymer aqueous emulsion, a second monomer emulsion, that comprises at least one second ethylenically unsaturated monomer and an initiator, is prepared and then polymerized to provide a polymeric adhesive. The second emulsion polymerization is preferably performed in the same vessel to polymerize at least a portion of the unreacted first ethylenically unsaturated monomer with at least a portion of the polymeric adhesive mixture. The second stage may be conducted, for example, by directly adding (e.g., all at once or by a gradual feed) the second monomer emulsion to the low molecular weight polymer aqueous emulsion. An advantage of this embodiment is that it is unnecessary to isolate the low molecular weight polymer, and the second polymerization can take place simply by adding the second monomer emulsion, that comprises at least one second ethylenically unsaturated monomer and initiator, to the low molecular weight polymer aqueous emulsion and then polymerizing to form the adhesive composition.
The adhesive compositions of the present invention may, optionally, further include other additives known in the art such as, for example, emulsifiers, pigments, fillers, curing agents, thickeners, humectants, wetting agents, defoamer, biocides, adhesion promoters, colorants, additional chemical classes of tackifying resins, UV stabilizers, waxes, antioxidants, and the like.
An adhesive article may be made by applying a coating of the adhesive composition of the present invention to a primary substrate and allowing the coating to dry, thereby providing an adhesive layer consisting of the solids portion of the adhesive composition covering a portion of the surface of the substrate.
The coating of adhesive composition can generally be applied to at least a portion of at least one surface of the primary substrate by any convenient method such as, for example, roll coating, wire-wound rod coating, slot die coating, gravure coating, knife coating, hot melt coating, or curtain coating, and allowed to dry to form a dry adhesive layer on the coated portion of the surface of substrate. The adhesive composition may also be applied as a continuous coating or a discontinuous coating on the surface of the primary substrate.
In one embodiment, the adhesive composition may be applied to a surface of the primary substrate in an amount effective to provide a dry adhesive layer 5 grams per square meter (g/m2) to 100 g/m2 on the coated portion of the surface of the primary substrate.
In a further embodiment, the primary substrate may be a flexible, sheet-like material such as, for example, a sheet of paper, a polymer film, a textile fabric or a nonwoven fiber sheet, and the adhesive article of the present invention is correspondingly a sheet-like material such as, for example, a pressure sensitive adhesive tape, a pressure sensitive adhesive label or a pressure sensitive adhesive film.
In a preferred embodiment, the adhesive article is an adhesive tape having an adhesive coated surface and an opposite non-coated surface. In a preferred embodiment, the article may include a release layer, e.g., a polymer film, for temporarily covering the adhesive layer prior to use. In an alternative preferred embodiment, wherein an adhesive tape is provided in the form of a concentrically wound roll, the non-coated surface of the underlying layer of tape functions as a release layer for the adhesive layer.
Alternately, the adhesive can be applied and dried on a release substrate (release liner paper) and dry transfer coated to primary substrates like paper or film labels.
The adhesive composition may be applied to more than one surface of the primary substrate, for example, both sides of a strip of a polymer film may be coated to make a xe2x80x9cdouble-sidedxe2x80x9d adhesive tape.
The adhesive articles of the present invention can be used by removing the release layer, if present, from the article and then applying an adhesive coated surface of the adhesive article to one or more secondary substrates or to one or more portions of a single secondary substrate to form a composite article wherein the substrates or primary substrate and secondary substrate portions are bonded together by an interposed dry adhesive layer.
Preferred secondary substrates include, but are not limited to, sheet-like materials such as, for example, paper products such as papers and paperboards, cardboards, corrugated cardboards, wood, metal films, polymer films and composite substrates. The terminology xe2x80x9ccomposite substratesxe2x80x9d as used herein means substrates consisting of a combination of dissimilar substrate materials such as polymer-coated paperboards or cardboards such, for example, wax-coated cardboard, and bonded wood products such as, for example, particle boards.
The invention is further described in the following examples. In the following examples, Examples 1 and 2 are actual examples. Examples 3 and 4 are prophetic examples. These examples are for illustrative purposes only.
In the examples, monomer conversion was determined by GC analysis of unreacted monomer using standard methods. Weight percent solids for polymer dispersions were determined by gravimetric analysis. Particle size of the polymer dispersions were obtained using a Matec CHDF 2000 particle size analyzer equipped with a HPLC type Ultra-violet detector. The Tg values measured using a differential scanning calorimeter (Model 12920, TA Company) by scanning the temperature range of xe2x88x92100xc2x0 C. to +150xc2x0 C. at a rate of 20xc2x0 C. per minute and reporting the midpoint of the Tg transition provided by the scan as the Tg value for the polymer.
Except where noted differently, the low molecular weight polymer was measured for number average molecular weight by SEC using a polystyrene standard from Polymer Laboratories (PS-1) having a peak average molecular weight ranging from 580 to 7,500,000 with narrow molecular weight distribution. Conversions from polystyrene to PMMA were made using Mark-Houwink constants. Copolymer compositions were evaluated for number average molecular weight and weight average molecular weight using SEC as described above.
The following abbreviations shown in Table 1 are used in the examples: